The secret of success is learning how to use pain and pleasure instead of having pain and pleasure use you. If you do that, you're in control of your life. If you don't, life controls you.

This quote means that if you develop the ability to direct your behavior based on what will be most pleasant or beneficial in the long-term, rather than being controlled by the immediate pleasure or pain of any particular situation, you will be successful in life. When you are in control, you make decisions that lead to success, rather than having life’s circumstances dictate what happens to you.